= Hertzsprung-Russell diagram
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A <Hertzsprung-Russell diagram> plots stellar <luminosity> against <effective temperature> or a calibrated <spectral classification>. Conventionally, high <luminosity> is upward and high <temperature> is to the left. The <Stefan–Boltzmann law>, $L=4\pi R_\star^2\sigma T_{\rm eff}^4$, explains why <stars> of equal <temperature> but different <luminosities> must have different <stellar radii>.
The <main sequence> crosses the diagram, <giant stars> occupy a luminous region, and <white dwarfs> lie at low <luminosity> despite their high <temperatures>. An evolutionary track follows one model <star> through this space; a population diagram instead combines many <stars> at once. A <colour-magnitude diagram> is the observational counterpart, requiring distance, <extinction> and spectral calibration for the conversion.
\Image[/stellar-astrophysics-hr-diagram.png]
{title=Schematic <Hertzsprung-Russell diagram> with lines of constant <stellar radius>}
{description=The <main sequence> curve and giant/dwarf positions illustrate regions rather than measured population data. The dashed lines follow the <Stefan–Boltzmann law>; the <Sun> normalizes the axes.}
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